This invention relates to the preparation of high strength zirconium alloys containing 7.0-10.0 wt % aluminum.
Structural materials for use in thermal nuclear reactors are required to satisfy a number of requirements, including:
1. Low absorption cross section for thermal neutrons.
2. High corrosion resistance to the gaseous or liquid reactor coolant.
3. Good mechanical strength and ductility.
4. Resistance to creep deformation at elevated temperatures.
5. Resistance to wear and erosion.
Zirconium alloys have been found to satisfy many of these requirements. Commonly employed has been the alloy known under the trade name of Zircaloy-2, and a Zr-2.5%Nb alloy.
Since the earliest days of zirconium technology it has been recognized that alloys based on the .alpha.Zr solid solution phase of the Zr-Al system, are superior to any other known group of Zr alloys in respect of their mechanical strength (see A. D. Schwope and N. Chubb Journal of Metals Vol. 4, 1942 page 1138). However, additions of Al of sufficient magnitude to improve the mechanical properties also have the disadvantage that they greatly reduce the corrosion resistance of the alloys, resulting in catastrophic oxidation in steam or water (see R. L. Carpenter, M. D. Carver and E. T. Hayes, U.S. Bureau of Mines Report USBM-U-40 (1955) and M. D. Carver and E. T. Hayes, USBM-U-122 (1956). Because of this difficulty Zr-Al alloys have been disregarded as possible construction materials for use in thermal nuclear reactors.
Zirconium alloys in current use, and many others which have been invented, are based on a matrix of zirconium which contains in solution one or more other elements, and in which matrix there may or may not be precipitates of compounds of zirconium and/or other elements.
The Zr-Al phase diagram was extensively studied by McPherson and Hansen and published in the Transactions of the American Society for Metals, Vol. 46, page 354, 1954. It was found that an intermediate phase, located close to 13% Al, and designated Zr.sub.2 Al exists between about 1200.degree. and 1300.degree. C. It is formed by the peritectoid reaction: .beta.Zr+Zr.sub.5 Al.sub.3 .revreaction.Zr.sub.2 Al. Another peritectoid reaction was found to occur between 950.degree. C. and 1,000.degree. C. involving the phases: .beta.Zr+Zr.sub.2 Al.revreaction.Zr.sub.3 Al (8.97% Al).
In the course of their work McPherson and Hansen prepared, from high-purity material, 25 gram buttons of Zr-alloys containing 7.0, 7.8, 9.4 and 9.7 wt% aluminum and subjected them to annealing treatments up to about 400 hours at less than approximately 950.degree. C. They determined that the microstructure of the 7.8 wt% Al alloys annealed at these temperatures was substantially that of isolated crystals of Zr.sub.3 Al dispersed in a matrix of transformed .beta.Zr containing Al in solution, implying that the alloy containing 7.0 wt% Al has a similar structure; the microstructure of the 9.4 and 9.7 wt% Al alloys was a mixture of Zr.sub.3 Al and Zr.sub.2 Al. Subsequently Keeler and Mallery (Transactions of American Inst. of Mining & Metallurgical Engineers, Feb. 1955, Page 394) reported the crystal structure and confirmed the earlier McPherson and Hansen report on the plastic properties of Zr.sub.3 Al. These workers did not, however, appreciate that Zr-Al alloys having a continuous or substantially continuous matrix of Zr.sub.3 Al have significantly different characteristics than Zr-Al alloys having a matrix of .alpha. or .beta. zirconium. Indeed, from the McPherson and Hansen observation (FIG. 9, p. 360) of crystals of Zr.sub.3 Al in a Zr matrix, the knowledgeable metallurgist would have suspected that such alloys would have very low corrosion resistance. No corrosion, tensile or creep properties of the 7.0-9.7 wt% Al alloys were determined due, it is believed, at least in part to the poor corrosion experience previously reported which taught away from the practical use of any Zr-Al alloy with a zirconium matrix. It was not in fact known whether an alloy could be formed having a continuous or substantially continuous matrix of Zr.sub.3 Al, or if possible, how such an alloy could be formed on a commercial scale.
Considering the McPherson and Hansen paper, page 360, FIGS. 8 and 9 (and captions therefor) and page 361, lines 5-8, it appears that the authors annealed their 7.8 wt% Al alloy to obtain "Predominant phase is Zr.sub.2 Al in a matrix of transformed beta". Thus the Zr.sub.2 Al would seem to be largely isolated coarse particles of Zr.sub.2 Al (see FIG. 8 and its caption). To obtain FIG. 9, the "same alloy" as in FIG. 8 was apparently annealed at 950.degree. C. for 45 hours to give "Crystals of Zr.sub.3 Al in a matrix of transformed beta".
From experimentation it has been found that annealing of alloys having coarse particles of Zr.sub.2 Al in a .beta.Zr matrix, for time periods up to about 160 hours at a temperature of about 950.degree. C., results in transformation of Zr.sub.2 Al to Zr.sub.3 Al only at the interface surrounding the initial Zr.sub.2 Al particles.
The layer of Zr.sub.3 Al surrounding the Zr.sub.2 Al phase apparently greatly slows the diffusion of Zr and the transformation to Zr.sub.3 Al and tends to isolate the Zr.sub.3 Al and prevent its forming the continuous matrix phase. It thus becomes clear that casting and initial annealing techniques should be controlled to avoid forming Zr.sub.2 Al phase as coarse dispersed particles. In small ingots, coarse Zr.sub.2 Al particles were found to inhibit the matrix phase Zr.sub.3 Al transformation, and in much larger ingots one could assume still coarser Zr.sub.2 Al particles would form and be even more of a problem.
Indeed it might well have been thought that in an ingot of sufficient size to be of practical value, the Zr.sub.2 Al particles would be so large that transformation to give a Zr.sub.3 Al matrix would be virtually impossible. However, we have shown that by control of ingot diameter and other casting parameters, ingots of 500 pounds can be subsequently transformed to a Zr.sub.3 Al matrix in reasonable times. This could certainly not be decuded from the work of McPherson et al, who used very small ingots and high purity material, and had no disclosure of avoiding Zr.sub.2 Al coarse particles which prevent obtaining a Zr.sub.3 Al matrix phase.